Re: [Xen-devel] [PATCH] x86-64 linux: unmap temporary mappings established for setup of 1:1 mappings

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 1 Jun 2006, at 19:21, Keir Fraser wrote:

> > The temporary mappings needed to set up the 1:1 mappings must be torn 
> > down after use; otherwise they may trigger the
> > WARN_ON() in vmap_pte_range() (namely if the chunk allocated to hold 
> > kernel and initial page tables gets close to or
> > exceeds 128Mb, or if a sufficiently high mem= argument causes the 
> > static allocations to grow beyond 128Mb, which in
> > either case means these mappings extend into the modules area).
> I've applied this to -unstable, but in this patch I only see code to 
> destroy the extended init mappings. What happens if you have a really 
> big initrd (for example)? That will push the initial pagetables up 
> into the modules area -- is that handled correctly now or is more 
> fixup required?
Might it make sense to zap all init mappings (after kernel code/data) 
in mem_init(), where we leave 'bootmem mode'? At that point initrd etc. 
can all be accessed via the main 1:1 mapping. I think this should work 
so long as we don't have any residual pointers to the init mapping 
hanging around.
Another concern I have is that our x86/64 mm/init.c is still very 
complicated and rather different from native mm/init.c (even though I 
did clean it up a lot some time ago). I'm interested in any ideas about 
how to reduce the complexity...
 -- Keir


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